Firewall is the wall that is a set between the defense network and exterior network in the event of the unexpected, destructible infections. By defecting, limiting and changing the data streams which are across the firewall, the firewall can do its best to shade the data, structure and running state of exterior network so as to realize the security of network. This article mainly discusses the working principle of firewall and puts forward the methods by which the firewall can be realized.%防火墙是设置在被保护网络和外部网络之间的一道屏障，以防止发生不可预测的、潜在破坏性的侵入.它可通过监测、限制、更改跨越防火墙的数据流，尽可能地对外部屏蔽网络内部的信息、结构和运行状况，以此来实现网络的安全保护.文中着重讨论防火墙的工作原理，提出几种防火墙的实现方式.
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